Metal-air electrochemical power sources, specifically zinc-air batteries and fuel cells with alkaline electrolyte are suitable for electric vehicles, unmanned aerial vehicles (UAVs), reserve, emergency power supply and other applications.
Zinc-air batteries (non-rechargeable) and zinc-air fuel cells, (mechanically-rechargeable) are electrochemical batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. In operation, a mass of zinc particles forms a porous anode, which is saturated with an alkaline electrolyte. Oxygen from the air reacts at the cathode and forms hydroxyl ions which migrate into the zinc paste (anode) and form zincate [Zn(OH)4]2−, releasing electrons to travel via the external circuit to the cathode. The zincate decays into zinc oxide and water returns to the electrolyte. The water from the anode is recycled at the cathode, so the water is not consumed.
The overall cell reaction is:2Zn+O2=2ZnO(E°=1.65 V)where, E° is the standard potential for the reaction. Theoretical specific energy according to the overall reaction equation is 1,350 Wh kg−1. The practical discharge voltage is about 1.15-1.05 V per cell depending on the current loading. Usually, current densities of 25-50 mA/cm2 are withdrawn.
Zinc-air batteries have some properties of fuel cells: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste.
Rechargeable zinc-air cells present a difficult design problem since zinc deposition from the water-based electrolyte must be closely controlled. The problems are dendrite formation, non-uniform zinc dissolution and limited solubility in electrolytes. Electrically reversing the reaction at a bifunctional air cathode, to liberate oxygen from discharged reaction products, is difficult; air electrodes tested to date are not robust and have low overall efficiency. Charging voltage is much higher than discharge voltage, producing cycle energy efficiency as low as 50%. Providing charge and discharge functions by separate uni-functional cathodes, increases cell size, weight, and complexity. A satisfactory electrically recharged system potentially offers low material cost and high specific energy.
Rechargeable systems may mechanically replace the anode and electrolyte each cycle, essentially operating as a refurbishable primary cell, or may use zinc powder or other methods to replenish the reactants. Mechanical recharging systems have been researched for decades for use in electric vehicles. Some approaches use a large zinc-air battery to maintain charge on a high discharge-rate battery used for peak loads during acceleration. Zinc granules serve as the reactant. Vehicle battery exchange swapped electrolyte and depleted zinc for fresh reactants at a service station to recharge.
In order to mechanically recharge a zinc-air battery, the spent zinc fuel (e.g. zinc electrolyte paste or slurry), formed during the electrochemical reaction, mainly in the form of zinc oxide, has to be removed from the cell. However, the spent zinc fuel forms a solid or a semi solid “cake” in the cell, mainly comprised of zinc oxide waste. Removing this “cake” quickly is difficult, and the remaining “cake” may damage the rest of the cell. Particularly, the air electrode or the cell separator can be damaged during the removal process. Furthermore, the capacity of the zinc-air cell is bounded by the amount of zinc that can be placed in about a 3 mm thick layer, as larger thicknesses of zinc may not be discharged efficiently. Efficient removal of spent zinc fuel from zinc-air cells and loading fresh zinc fuel into the zinc-air cell, will enable performing multiple cycles of charge/discharge of a cell/battery where the energetic capacity of such a system will be determined by the amount of zinc fuel in the system.